U.S. Pat. No. 5,588,803 describes some of the basic structural and manufacturing issues involved in producing molded plastic centrifugal fans for automotive air conditioning systems. The ultimate in molding simplicity is a one piece design, which can be made only by designing the fan with a shape that is amenable to the so called axial draw or by pass molding technique. In order to be moldable by that technique, the part, be it a fan or anything else, must have a certain structural relationship relative to its central axis, such as the central axis of a bearing cage or the central axis of a fan. All "upper" and "lower" surfaces of the part must be divisible in such a way that they have no radial overlap with one another. If so designed, all part surfaces may be divided up so that some can be molded by one die, and the rest by the other die, and the pair of dies (or molds) can be pushed together and pulled apart freely along the same central axis. This represents the absolute minimum both in terms of the number of molds used (two) to produce the part, and the number of pieces (one) in the part produced.
A dilemma is faced in designing a centrifugal fan with such a "no radial overlap" design, especially for so called rearwardly inclined fan blade designs, which are wide in the radial direction. Both the lower blade bases and the upper blade tips need adequate structural support. The blade bases may be easily integrally molded to the central area of the fan, which has a thick center hub. However, to provide complete support to the upper tips of the blades, an upper ring is needed, which is axially spaced from the hub, and inevitably overlaps with it. The issue then becomes the best way to physically attach this non integral blade tip supporting ring. This may be done by separate fasteners, heat staking, or, as in the above referenced patent, by a twist lock technique.
This is not to say that it's impossible to by pass mold a plastic centrifugal fan, even one with radially wide blades. A design capable of being molded that way is relatively simple, and an example of such a design is disclosed in U.S. Pat. No. 5,352,089. The design involves basically splitting off the radially outermost section of the hub at an imaginary cylindrical line and moving it up to support the tips of the fan blades. Then, the two molds can part along that imaginary cylinder, which is arrayed around the central axis. Inevitably, the entire width of the base and tips of the blades cannot both be structurally supported, however. Only the radially inner portions of the base of the fan blades are supported, by the hub, and the radially outer portions are unsupported by the hub. Likewise, only the radially outer portions of the tips of the blades are supported, by the upper rim, and the radially inner portions are unsupported. Sufficient structural stiffness can be achieved simply by making the hub, rim and blades thick enough, of course.
However, in a two piece fan design, the hub at the blade bases, and the radially overlapped ring at the blade tips, provide more than just blade stiffness. The air that is pulled axially in and then driven radially outwardly between the blades is also confined between the axially opposed lower hub and upper ring. The upper ring generally slopes axially downwardly relative to the lower hub (to maintain a constant volume as the radius increases), and both the hub and ring generally slope axially downwardly relative to the air capturing, torroidal volute that surrounds the fan. With a by pass molded, one piece fan design, both the hub and upper ring are "incomplete," and cannot alone do an efficient job of confining the radially outwardly moving air stream. For example, in the design disclosed in U.S. Pat. No. 5,352,089, the unsupported outer portions of the blade bases are simply left wide open, decreasing the effectiveness of the fan assembly as a whole.